SNVS037H September   1999  – March 2024 LM1084

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Pin Configuration and Functions
  6. 5 Specifications
    1. 5.1 Absolute Maximum Ratings
    2. 5.2 ESD Ratings
    3. 5.3 Recommended Operating Conditions
    4. 5.4 Thermal Information
    5. 5.5 Electrical Characteristics
    6. 5.6 Typical Characteristics
  7. 6 Detailed Description
    1. 6.1 Overview
    2. 6.2 Functional Block Diagram
    3. 6.3 Feature Description
      1. 6.3.1 Ripple Rejection
      2. 6.3.2 Load Regulation
      3. 6.3.3 Overload Recovery
    4. 6.4 Device Functional Modes
      1. 6.4.1 Output Voltage
      2. 6.4.2 Stability Consideration
      3. 6.4.3 Protection Diodes
  8. 7 Application and Implementation
    1. 7.1 Application Information
    2. 7.2 Typical Applications
      1. 7.2.1  1.2-V to 15-V Adjustable Regulator
        1. 7.2.1.1 Design Requirements
        2. 7.2.1.2 Detailed Design Procedure
        3. 7.2.1.3 Application Curve
      2. 7.2.2  Adjustable at 5 V
      3. 7.2.3  5-V Regulator With Shutdown
      4. 7.2.4  Battery Charger
      5. 7.2.5  Adjustable Fixed Regulator
      6. 7.2.6  Regulator with Reference
      7. 7.2.7  High Current Lamp Driver Protection
      8. 7.2.8  Battery Backup Regulated Supply
      9. 7.2.9  Ripple Rejection Enhancement
      10. 7.2.10 Automatic Light Control
      11. 7.2.11 Generating Negative Supply Voltage
      12. 7.2.12 Remote Sensing
    3. 7.3 Power Supply Recommendations
    4. 7.4 Layout
      1. 7.4.1 Layout Guidelines
        1. 7.4.1.1 Thermal Considerations
      2. 7.4.2 Layout Example
  9. 8 Device and Documentation Support
    1. 8.1 Receiving Notification of Documentation Updates
    2. 8.2 Support Resources
    3. 8.3 Trademarks
    4. 8.4 Electrostatic Discharge Caution
    5. 8.5 Glossary
  10. 9 Revision History
  11. 10Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

7.4.1.1 Thermal Considerations

ICs heats up when in operation, and power consumption is one factor in how hot it gets. The other factor is how well the heat is dissipated. Heat dissipation is predictable by knowing the thermal resistance between the IC and ambient (θJA). Thermal resistance has units of temperature per power (C/W). The higher the thermal resistance, the hotter the IC.

The LM1084 specifies the thermal resistance for each package as junction to case (θJC). In order to get the total resistance to ambient (θJA), two other thermal resistance must be added, one for case to heat-sink (θCH) and one for heatsink to ambient (θHA). The junction temperature can be predicted as follows:

Equation 3. TJ = TA + PD (θJC + θCH + θHA) = TA + PD θJA

TJ is junction temperature, TA is ambient temperature, and PD is the power consumption of the device. Device power consumption is calculated as follows:

Equation 4. IIN = IL + IG
Equation 5. PD = (VIN−VOUT) IL + VINIG

Figure 7-13 shows the voltages and currents which are present in the circuit.

GUID-843D52FA-ECE5-42DF-A51A-A5831B79DE99-low.png Figure 7-13 Power Dissipation Diagram

Once the devices power is determined, the maximum allowable (θJA (max)) is calculated as:

Equation 6. θJA (max) = TR(max)/PD = TJ(max) − TA(max)/PD

The LM1084 has different temperature specifications for two different sections of the IC: the control section and the output section. The Thermal Information table shows the junction to case thermal resistances for each of these sections, while the maximum junction temperatures (TJ(max)) for each section is listed in the Absolute Maximum Ratings table. TJ(max) is 125°C for the control section, while TJ(max) is 150°C for the output section.

θJA (max) should be calculated separately for each section as follows:

Equation 7. θJA (max, CONTROL SECTION) = (125°C - TA(max))/PD
Equation 8. θJA (max, OUTPUT SECTION) = (150°C - TA(max))/PD

The required heat sink is determined by calculating its required thermal resistance (θHA (max)).

Equation 9. θHA (max) = θJA (max) − (θJC + θCH)

(θHA (max)) should also be calculated twice as follows:

Equation 10. (θHA (max)) = θJA (max, CONTROL SECTION) - (θJC (CONTROL SECTION) + θCH)
Equation 11. (θHA (max)) = θJA(max, OUTPUT SECTION) - (θJC (OUTPUT SECTION) + θCH)

If thermal compound is used, θCH can be estimated at 0.2 C/W. If the case is soldered to the heat sink, then a θCH can be estimated as 0 C/W.

After, θHA (max) is calculated for each section, choose the lower of the two θHA (max) values to determine the appropriate heat sink.

If PC board copper is going to be used as a heat sink, then Figure 7-14 can be used to determine the appropriate area (size) of copper foil required.

GUID-7753917E-8779-4C81-976B-0739CD3C9E63-low.png Figure 7-14 Heat Sink Thermal Resistance vs Area